The present invention relates to a capacitor element for a power capacitor including a plurality of films of dielectric material, forming two adjacent dielectric layers, and also a plurality of electrodes of metal material, at least two such electrodes being situated between the two dielectric layers spaced from and alongside each other to define an area that is free from metal material.
The invention also relates to a method for manufacturing such a capacitor element, and a power capacitor comprising such a capacitor element.
In this context power capacitors refers to capacitors for alternating or direct current applications for voltages exceeding 1 kV, preferably at least 5 kV.
It is known to use self-healing films, with or without inner series connections, in capacitor elements for power capacitors for alternating and direct current applications. Such capacitor elements are known, for instance, from EP 0 225 822 B1, according to which the self-healing films have electrodes of varying thickness, and BP 0 789 371 A1, according to which the self-healing films have segmented, series-connected electrodes. Elongated films of a dielectric material are used in the production of such capacitor elements, said material being partially coated with a metal material to form one or more electrodes. Production entails laying a plurality, usually two, of such coated films one on top of the other and winding them to a roll so that the electrodes are capacitively connected together. A plurality of capacitor elements are coupled together in series and in parallel, and enclosed in a container, to produce the actual power capacitor. In order to increase the filling ratio of the power capacitor, the capacitor elements are in certain cases flattened before being coupled together and placed in the container. The dielectric material in the films of the capacitor element is usually polypropene or polythene terephthalate and the electrode material is usually aluminium, zinc or an alloy of these two.
The self-healing properties are obtained through selection of the dielectric films and electrodes so that, upon an electric discharge through any of the films, the electrodes are vaporized locally around the fault point, thus isolating the fault point electrically. The electrodes must be thin if the self-healing is to function, and the electrode material is therefore usually vaporized onto the film to form the electrode layers. The self-healing is most efficient if the dielectric films are thinner than about 15 micrometer, which means that the voltage across the films cannot be permitted to be very high. However, through internal series connection, it is possible to increase the voltage across the capacitor element without increasing the voltage across the films.
In order to achieve the internal series connection, the capacitor element has two or more electrodes that are arranged between two adjacent films, which electrodes are electrically insulated from each other by means of uncoated parts of the films. When the capacitor element is placed under voltage, these electrodes acquire different potentials so that voltage gradients occur along the boundary layer of the films between adjacent electrodes. If a flashover occurs between two adjacent electrodes this may short-circuit the capacitor element, making the capacitor element unusable. If the energy in the capacitor element is considerable such a short circuit may also damage adjacent capacitor elements and cause considerable damage to the power capacitor in which the capacitor element is included. To obtain the necessary electric strength, therefore, the width of the uncoated parts must be dimensioned so that flashover between the electrodes does not occur. Since the electric strength is considerably lower along the longitudinal surface of a film than transversely, substantial safety margins must be used for this dimensioning.
The requirement described above for high electric strength between electrodes with different potentials is, however, in conflict with the desire to minimize the area of the uncoated parts to increase utilization of volume and material in the capacitor element. A.usual measure for increasing the electric strength between the electrodes is to completely or partially impregnate the capacitor element with a suitable impregnating fluid. The electric strength increased in this way can be utilized to decrease the area of the uncoated parts and/or increase the permissible voltage across the capacitor element. Generally, however, it is desirable to avoid impregnating fluid because of the risk of leakage and fire, environmental aspects, technical manufacturing aspects and so on.
The object of the present invention is to create a capacitor element which, in impregnated or unimpregnated state, offers better volume and material utilization and/or withstands a higher voltage than equivalent known impregnated or unimpregnated capacitor elements.
The capacitor element in accordance with the invention is characterized in that a permanent connection of a dielectric material is arranged in said area and unites the dielectric layers with each other.
The method in accordance with the invention is characterized in that said dielectric layers are united within said area by means of a permanent connection of a dielectric material.
The power capacitor in accordance with the invention is characterized in that a permanent connection of a dielectric material is arranged in said area and unites the dielectric layers with each other.
Improved electric strength in said area is obtained through the invention. This can be utilized to decrease the size of the area, thus contributing to better volume and material utilization in the capacitor element, and thus in the power capacitor. Alternatively, the improved electric strength may be used to increase the voltage over the capacitor element. Furthermore, thanks to the increased electric strength, unimpregnated instead of impregnated capacitor elements can be used in certain applications.
In accordance with one embodiment said connection is formed by one of said plurality of films.
In accordance with another embodiment of the invention the permanent connection is achieved by fusion of a plurality of said films in said area, which leads to the boundary layer between the films, which is sensitive from the electric strength aspect, completely or partially disappears.
In accordance with one embodiment the power capacitor according to the invention comprises a plurality of capacitor elements, that have substantially circular-cylindrical shape, are arranged close together so that their axial directions coincide, and are connected to each other so that they form a series-connected capacitor stack. In such a power capacitor for high voltage the technique of using inner series-connections in the capacitor elements is an obvious advantage since the number of series-connected capacitor elements can be reduced. The technique is particularly advantageous together with the technique mentioned above for self-healing. Since successful self-healing requires particularly thin metal coating and the currents flowing through the metal generate active power dissipation (heat), thinner layers result in higher losses. One way of reducing the losses without compromising the requirement for a thin metal coating is to choose a shape for the metallized film, and thus a shape for the capacitor element, such that the dimension of the metal coating perpendicular to the rolling direction is decreased and the length of the roll is increased. Unless internal series-connection is used, the consequence of this will be that the cylindrical capacitor elements acquire a relatively little height in relation to their diameter. Series-connecting many such elements, which is required for high voltage, becomes detrimental from the cost point of view. With inner series connections, therefore, several series-connected part-capacitors can automatically be built into a cylindrical capacitor element with an optimal relation between height and diameter, from the manufacturing aspect, and with good self-healing properties.